With the advancement of ubiquitous network society, there is a large demand for mobile devices such as cellular phones, notebook personal computers, and digital still cameras. As the power source for mobile devices, it is desired to put fuel cells, which do not have to be recharged and can continuously supply power to devices if get refueled, into practical use as early as possible.
Among fuel cells, direct oxidation fuel cells, which generate power by directly supplying an organic fuel such as methanol or dimethyl ether to an anode for oxidation without reforming it into hydrogen, are actively studied and developed. Direct oxidation fuel cells are receiving attention in terms of the high theoretical energy densities of organic fuels, system simplification, ease of fuel storage, etc.
A direct oxidation fuel cell has a unit cell composed of a membrane-electrode assembly (hereinafter referred to as an MEA) sandwiched between separators. The MEA is composed of a solid polymer electrolyte membrane sandwiched between an anode and a cathode, and each of the anode and the cathode includes a catalyst layer and a diffusion layer. Such a direct oxidation fuel cell generates power by supplying a fuel and water to the anode and supplying an oxidant to the cathode.
For example, the electrode reactions of a direct methanol fuel cell (hereinafter referred to as a DMFC), which uses methanol as the fuel, are as follows.Anode: CH3OH+H2O→CO2+6H++6e−Cathode: 3/2O2+6H++6e−→3H2O
On the anode, methanol reacts with water to produce carbon dioxide, protons, and electrons. The protons migrate to the cathode through the electrolyte membrane, and the electrons migrate to the cathode through an external circuit. On the cathode, these protons and electrons combine with oxygen gas to form water.
However, practical utilization of DMFCs has some problems.
One of the problems relates to durability. The power generating performance of DMFCs degrades in an initial stage, and the main cause of this initial degradation is assumed to be an increase in cathode overvoltage. The increase in cathode overvoltage occurs probably because water accumulates inside the catalyst layer of the cathode or at the interface between the catalyst layer and the diffusion layer of the cathode with time of power generation, thereby lowering the diffusibility of oxygen gas. Further, this initial degradation is largely affected by methanol crossover (hereinafter referred to as MCO), which is a phenomenon of permeation of unreacted methanol through the electrolyte membrane to the cathode. An increase in MCO causes an increase in cathode activation overvoltage. Further, carbon dioxide gas produced by oxidation reaction of such crossover methanol further lowers the diffusibility of oxygen gas. As a result, the power generation performance degrades significantly.
Such initial degradation tends to occur in the power generating region opposing the upstream part of the fuel flow channel where a large amount of MCO occurs, and is evident particularly when the concentration of oxygen gas at the part of the oxidant flow channel opposing the upstream part of the fuel flow channel is low. That is, it is thought that the initial degradation tends to occur when the flow direction of fuel is opposite to the flow direction of oxidant. It should be noted that direct oxidation fuel cells are usually structured so that the flow direction of fuel is opposite to the flow direction of oxidant in the same manner as fuel cells using hydrogen gas as the fuel. Such structure is intended to make the current density uniform in the power generating region where an anode and a cathode oppose each other with an electrolyte membrane therebetween in consideration of the balance of overvoltage.
One approach to reducing the initial degradation is to supply a large amount of oxidant to the cathode. However, this approach requires upsizing of an oxidant supply means such as an air pump or blower, and the upsizing requires additional electric power. Further, if the amount of oxidant supply is increased too much, the polymer electrolyte in the solid polymer electrolyte membrane and catalyst layers of the MEA becomes dry, so that the proton conductivity lowers. As a result, the power generating characteristics degrade significantly.
Also, with respect to the flow direction of fuel and the flow direction of oxidant, a large number of proposals have been made for solid polymer electrolyte fuel cells (hereinafter referred to as PEFCs).
For example, Japanese Laid-Open Patent Publication No. 2006-02570 (Document 1) discloses a fuel cell in which the opposing flow region where the flow direction of fuel through the fuel flow channel is opposite to the flow direction of oxidant through the oxidant flow channel accounts for not less than 70% of the power generating region, provided that the projected area of the power generating region is 100%.
Japanese Laid-Open Patent Publication No. 2002-184428 (Document 2) discloses a fuel cell in which the laminating direction of its unit cells is perpendicular to the gravity direction and inlets and outlets of fuel gas and oxidant gas and gas flow channels are arranged so that the humidity distribution in the anode-side reaction area is opposite to the humidity distribution in the cathode-side reaction area.
However, the above-described conventional art cannot suppress an increase in overvoltage in the part of the cathode-side power generating region opposing the upstream part of the fuel flow channel where a large amount of MCO occurs. It is thus difficult to provide a direct oxidation fuel cell with excellent durability.
As in the techniques represented by Documents 1 and 2, in order to make the water distribution in the power generating region uniform, the flow direction of fuel and the flow direction of oxidant can be made opposite to circulate the moisture inside the power generating region. However, such techniques cannot provide drastic measures to suppress an increase in overvoltage in the part of the cathode-side power generating region opposing the upstream part of the fuel flow channel.
The invention solves the above-discussed problems associated with conventional art and intends to provide a direct oxidation fuel cell with excellent durability.